Methods and apparatus for improved low current AC/DC TIG welding and starting

ABSTRACT

A TIG welder and methodologies for providing an output welding current in a welding circuit. The welder has main and background power supplies. The main supply has an SCR network which selectively connects a transformer secondary winding to the welding circuit according to SCR control signals. The background supply is connected to the SCR network, which selectively connects the second power supply to the welding circuit according to the SCR control signals. The SCR network may have first and second SCRs operating according to first and second SCR control signals from a control circuit, where the control circuit is connected to the SCR network and the second power supply and selectively connects the SCR control signals to the SCR network according to the setpoint current value.

FIELD OF THE INVENTION

The present invention relates generally to welding equipment and moreparticularly to apparatus and methods for AC and DC TIG welding andstarting at low current levels.

INCORPORATION BY REFERENCE

Tungsten inert gas (TIG) welding involves the provision of AC or DCwelding current to a welding circuit in which a tungsten electrode isspaced from a workpiece to define a gap, in order to create an arc inthe gap for melting a filler wire moved into the arc. In TIG welding,problems occur in starting or establishing an arc to begin the weldingprocess, as well as in operating at relatively low currents. Withrespect to arc starting, De Coster U.S. Pat. No. 6,075,224 illustratesan arc starter circuit using high frequency and is incorporated hereinby reference as background information. In regards to DC weldingoperation at low current levels, auxiliary or background power suppliesare sometimes employed in conjunction with a standard phase controlledconstant current stage. In such welders, the background supply maintainsa minimum current and the main power supply provides weld current abovea minimum level. Examples of such welder architectures are illustratedin Campiotti U.S. Pat. No. 4,950,864, Vogel U.S. Pat. No. 5,218,182,Terayama U.S. Pat. No. 5,645,741, and Heraly U.S. Pat. No. 6,034,350,and these patents are incorporated herein by reference. In addition,Samodell U.S. Pat. No. 6,388,232 entitled “STARTING AND WELDING DEVICEFOR DC TIG WELDER AND METHOD OF OPERATING SAME”, assigned to theassignee of the present invention, is incorporated by reference as iffully set forth herein.

BACKGROUND OF INVENTION

Electric welders are employed in a variety of field applications, inwhich electric power is applied to a gap in a welding circuit between aworkpiece to be welded and an electrode. One type of welding process isknown as tungsten inert gas (TIG) welding, wherein heat is generatedfrom an electric arc maintained between a non-consumable tungstenelectrode and a part or workpiece being welded. Additional filler metalmay, but need not, be employed, such as a separate filler metal wire,where additional material is desired. In TIG welding operations, ashield of inert gas, typically argon protects the melt puddle,electrode, and the optional filler rod from the ambient atmosphere, inorder to prevent rapid oxidation of the weld and surrounding metal. TIGwelder power supplies may be AC, DC, or combinations thereof, asdetermined according to the type of metal to be welded. DC welding isoften used to weld stainless steel and mild and low alloy steels,whereas AC is typically used to weld aluminum. In AC welding, surfaceoxidation is removed in the half power cycle where the electrode ispositive, and hence this is referred to as the “cleaning” half-cycle,while the negative half-cycle is referred to as the “penetration”half-cycle. The welding voltages and currents provided during thepenetration and cleaning half-cycles are typically not equal, forinstance, wherein more energy is applied by the welder duringpenetration than in cleaning. TIG welders, both DC and AC types,commonly include arc starting systems providing high frequency power tothe welding circuit during arc initiation, which may be deactivated oncethe arc is established.

In DC TIG welding, currents are often provided to the welding circuitusing a single phase SCR rectifier, wherein the welding current isadjustable by varying the phase firing angle of one or more SCR devicesin an SCR network in an output rectifier. In industrial DC weldingapplications, currents as high as 200-300 amperes are common. However,in some applications, the same industrial DC welders are required tooperate at much lower current levels, for example, such as 5-10 amps orless used in welding thin aluminum workpieces. In such a situation, theSCR is only actuated for a short time near the end of the positive powersource half-cycle. As a result, arc stability at such low current levelssuffers. An auxiliary or background supply may be employed at such lowcurrent levels in order to create a fixed minimum current, inconjunction with the phase controlled rectifier. In this implementation,however, problems have been found in starting or establishing the arc atsuch low current levels. Samodell U.S. Pat. No. 6,388,232 assigned tothe assignee of the present application addresses the above shortcomingswith respect to DC welders being started and operated at current levelsof 5-10 amps or less.

In AC welding applications, alternating currents of 200-400 amperes areoften supplied to the welding circuit via SCR controlled square wavesupplies, having separate SCRs for connecting the welding electrode withpositive and negative voltages. In such AC welders, the SCRs are gatedor activated by gating signals during the positive or negativehalf-cycles of an AC supply source, where the portion of the supplyhalf-cycle in which the SCR is gated determines the amplitude of the ACcurrents in the welding circuit. Thus, this type of AC welder may alsoemploy phase firing angle control of SCR gating signals in order toprovide adjustable AC welding currents. As with DC welders, AC weldingequipment often is needed to operate at low current levels for certainapplications, and much higher currents for other applications.Difficulties arise in starting and operating such equipment at the lowend of the current range, for example, wherein operating levels belowabout 15-20 amps are desired.

Conventional AC and AC/DC welders often experience erratic operation atsuch low currents, wherein arc “popping” and “dancing high frequency”conditions are found. For example, a constant current (CC) square waveTIG (SWT) welder designed to operate at up to 200-400 amps controls theamount of AC current delivered to the welding circuit by varying thetime that the control SCRs are in the conducting state using phase anglefiring signals applied to the SCR control gates. Operation at lowcurrent levels requires the control signal to the SCR to be assertedvery late in the power cycle, and for a very short time. As the“on-time” for the SCR is reduced, a point is reached at which the shortcurrent pulse cannot be sustained by the output current choke in thewelder to maintain continuous current flow at high enough output voltageuntil the following half-cycle gate firing. In this situation, the lowcurrent “pops” out, and the arc stabilizing high frequency system isactivated to maintain the TIG arc ionization, thereby causing highfrequency arc “dancing”.

Heretofore, no AC or AC/DC SCR driven welding equipment has beendeveloped which can support both high and low current levels without theabove mentioned difficulties in starting and operating at low currentlevels. Thus, there remains a need for improved apparatus andmethodologies for starting and operating AC and AC/DC welders at lowcurrent levels, by which the above and other difficulties may be avoidedor mitigated.

SUMMARY OF INVENTION

A summary of one or more aspects of the invention is now provided inorder to provide a basic understanding of one or more aspects thereof.This summary is not an extensive overview of the invention, and isintended neither to identify key or critical elements of the invention,nor to delineate the scope of the invention. Rather, the primary purposeof the summary is to present some concepts of the invention in asimplified form prior to the more detailed description that is presentedlater. The present invention is directed to improvements in AC and AC/DCTIG welders by which low current starting and welding operation can beachieved without arc “popping” and “dancing” conditions associated withprior devices. The invention provides square wave AC welding voltagesfrom a background supply to fill in periods between SCR firings of amain AC supply, which advantageously uses an SCR network of the mainsupply. The background supply thus prevents or minimizes theintermittent loss of arc experienced in conventional welders, andreduces or avoids situations in which the high frequency startingcircuit is activated in low current welding operation. This allowswelding operation using the background supply alone at very low currentsnot heretofore achievable, as well as operation with both the backgroundand main supplies at higher currents. The apparatus provided furtherallows for a variety of arc starting techniques to be employed in lowcurrent startup situations.

One aspect of the invention provides a TIG welder operable to provide anoutput welding current across a gap in a welding circuit between anelectrode and a workpiece according to a setpoint current value. Thewelder comprises first and second (e.g., main and background) powersupplies, wherein the first power supply comprises an SCR networkoperable to selectively connect a transformer secondary winding to thewelding circuit according to SCR control signals so as to provide afirst current to the welding circuit. The second power supply isconnected to the SCR network, which operates to selectively connect thesecond power supply to the welding circuit according to the SCR controlsignals so as to provide a second current to the welding circuit throughat least one SCR in the first power supply. The SCR network may comprisefirst and second SCRs operating according to first and second SCRcontrol signals from a control circuit. The control circuit is connectedto the SCR network and the second power supply, which operates toselectively connect the SCR control signals to the SCR network accordingto the setpoint current value. The welder may be used to provide AC orDC current to the welding circuit according to the selected weldingprocess (e.g., for example, according to the type of metal beingwelded), wherein the welder may be configured (e.g., such as byappropriate jumpers or switches) to provide the corresponding AC or DCcurrents to the welding circuit.

The second power supply may comprise first and second backgroundsupplies connected to the SCR network, where the first SCR operates toselectively connect the first background power supply to the weldingcircuit according to the first SCR control signal, and the second SCRoperates to selectively connect the second background power supply tothe welding circuit according to the second SCR control signal so as toprovide the second current to the welding circuit. The connection of thebackground supplies to the SCR network advantageously allows one or moreof the SCRs therein to be maintained or latched in a conducting state bybackground current. This, in turn, facilitates filling in the periodswhere the main supply is not supplying current to the welding circuitwith background current. The background supplies may each comprise a DCsupply such as a rectifier connected to a dedicated transformersecondary in the welder, and a background control switch for selectivelyconnecting the rectifier to the SCR network according to a backgroundcontrol signal from the control circuit.

The control circuit may comprise a logic circuit connected to the firstand second background power supplies and the SCR network, as well as acomparator circuit connected to the logic circuit. The logic circuitprovides the first and second background control signals according tofirst and second SCR control signals, respectively, and according to adisable signal, for example, wherein the SCR control signals are phaseangle firing signals, and the disable signal is provided by thecomparator circuit according to the setpoint current value. The logiccircuit also provides first and second SCR gating signals to the firstand second SCRs according to the first and second SCR control signals,respectively, and according to the disable signal. The logic circuit,moreover, may selectively refrain from providing the first SCR gatingsignal and the first background control signal or refrain from providingthe second SCR gating signal and the second background control signal,in accordance with the disable signal. In this manner, the logic circuitmay provide operation of the welder with only one of the backgroundsupplies providing the welding circuit current, with the most recentlyfired SCR being latched by the background supply current. Thus, theinvention provides for extremely low current operation in a DC mode orin an AC mode for very low setpoint current values.

Another aspect of the invention provides a method of providing an outputwelding current in a welder, comprising providing a first power supplyhaving a first transformer secondary winding and an SCR network, the SCRnetwork being connected to the welding circuit, providing a second asecond power supply connected to the SCR network, selectively connectingthe first transformer secondary winding to the welding circuit using theSCR network during a first portion of a power cycle to provide a firstcurrent to the welding circuit, and selectively connecting the secondpower supply to the welding circuit during a remaining portion of thepower cycle using the SCR network. Selectively connecting the firsttransformer secondary winding may comprise operating at least one SCR ina conductive state according to the SCR control signal so as to connectthe first transformer secondary winding to the welding circuit duringthe first portion of the power cycle, and selectively connecting thesecond power supply may comprise latching the SCR using current from thesecond power supply so as to maintain the SCR in the conductive stateduring the remaining portion of the power cycle.

Yet another aspect of the invention provides a method for starting awelding arc in an AC/DC TIG welder. The method involves starting the arcusing DC current from the TIG welder and providing DC current thereafteruntil a current setpoint exceeds a reference value, such as about 7 ampsor less. The method comprises selectively gating a first SCR for aportion of a first input half-cycle to connect the first transformersecondary winding to the welding circuit. This provides current of afirst polarity to the welding circuit during the portion of the firstinput power half-cycle. The method further comprises connecting thesecond power supply to the welding circuit through the first SCR toprovide current of the first polarity thereto during the portion of thefirst input power half-cycle and thereafter until a second SCR in theSCR network is gated into a conductive state. In this fashion, currentfrom the second power supply latches the first SCR in the on state toprovide current of the first polarity to the welding circuit after thefirst input half-cycle ends. The method also comprises gating the secondSCR after a setpoint current value exceeds a first given value for aportion of a second input power half-cycle, thus connecting the firsttransformer secondary winding to the welding circuit to provide currentof a second polarity thereto during the portion of the second inputpower half-cycle. Thereafter, the first SCR may again be gated, and theprocess repeated to provide alternating (AC) welding current, until thesetpoint current value no longer exceeds the first given value.

The following description and drawings set forth in detail certainillustrative implementations of the invention. These are indicative ofbut a few of the various ways in which the principles of the inventionmay be employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary welder connectedfor AC welding operation in accordance with the present invention;

FIG. 2A provides voltage and current curves illustrating operation ofthe welder of FIG. 1 for low AC setpoint current values at about 5 amps;

FIG. 2B provides voltage and current curves illustrating operation ofthe welder of FIG. 1 for low AC setpoint current values above about 5amps;

FIG. 2C provides voltage and current curves illustrating operation ofthe welder of FIG. 1 for high AC setpoint current values;

FIG. 3 is a schematic diagram illustrating the exemplary welder of FIG.1 connected for DC(−) welding operation in accordance with anotheraspect of the present invention;

FIG. 4A provides voltage and current curves illustrating operation ofthe welder of FIG. 3 for low DC setpoint current values at about 2 amps;

FIG. 4B provides voltage and current curves illustrating operation ofthe welder of FIG. 3 for low DC setpoint current values above about 2amps;

FIG. 4C provides voltage and current curves illustrating operation ofthe welder of FIG. 3 for high DC setpoint current values;

FIG. 5 is a schematic diagram illustrating further details of thecontrol circuit of the exemplary welder of FIGS. 1 and 3;

FIG. 6 is a timing diagram illustrating various I/O signals provided bythe control circuit of the exemplary welder of FIGS. 1 and 3;

FIG. 7 provides a voltage curve illustrating output voltages from thefirst and second power supplies of FIG. 1 in AC mode operation;

FIG. 8 provides a voltage curve illustrating a composite AC outputvoltage provided by the first and second power supplies of FIG. 1;

FIG. 9 provides a voltage curve illustrating a composite DC outputvoltage provided by the first and second power supplies of FIG. 3 forintermediate or high current DC welding operation;

FIG. 10 provides a voltage curve illustrating a composite DC outputvoltage provided by the second power supply of FIGS. 1 and 3 for lowestcurrent DC(+) welding operation;

FIG. 11 is a schematic diagram illustrating further details of one ofthe exemplary background power supplies of the welder of FIGS. 1 and 3;

FIG. 12 provides voltage and current curves illustrating an exemplarymethod of low current DC(+) arc starting and welding with higher currenttransition to AC welding in accordance with another aspect of thepresent invention; and

FIG. 13 is a side elevation view in section illustrating a crater fillweld using the exemplary welder of the invention.

PREFERRED EMBODIMENT

One or more embodiments or implementations of the present invention willnow be described with reference to the drawings, wherein like referencenumerals are used to refer to like elements throughout. Apparatus andmethodologies are provided for providing AC or DC welding current, bywhich low current operation and arc starting may be facilitated inwelders capable of providing welding currents in excess of 100 amps. Theemployment of one or more background power supplies operativelyconnected to a main power supply SCR network allows stable AC or DCwelding operation, for example, at AC currents as low as about 5 amps orless, or at DC welding currents of about 2 amps or less, without theerratic low current operation typical in previous welders. In addition,the invention allows employment of a variety of arc starting procedureswhich further facilitate low current welding operation in welderscapable of much higher currents.

FIGS. 1 and 3 illustrate an exemplary AC/DC TIG welder 2 in accordancewith the present invention, which may be selectively operable to provideAC or DC weld currents to a welding circuit, through selection ofappropriate switch conditions of an AC/DC switch 3, implemented in thewelder 2 as a series of jumpers. Although the AC/DC switch 3 isillustrated and described herein as a series of jumpers, otherimplementations of the switch 3 are contemplated within the scope of thepresent invention, including relays, solid state switching devices, orothers as are known. In addition, while illustrated in the context ofthis exemplary TIG welder 2, it will be appreciated by those skilled inthe welding arts that the various aspects of the invention may becarried out in a variety of TIG welder architectures apart from thoseillustrated and described herein, and that such implementations arecontemplated as failing within the scope of the appended claims.

In FIG. 1, the exemplary welder 2 is illustrated comprising first andsecond (e.g., main and background) power supplies 4 and 6, respectively,wherein the first power supply 4 comprises an SCR network including apair of first SCRS 8 a and 8 b (hereinafter collectively referred to as8), and a pair of second SCRs 10 a and 10 b (hereinafter 10collectively). The welder 2 is operable to provide an output weldingcurrent 12 across a gap in a welding circuit between an electrode 14 anda workpiece 16 according to a setpoint current value I_(SET) in a TIGwelding operation. The first power supply 4 of the welder 2 alsocomprises a first transformer secondary winding XM associated with aprimary winding X1 for receiving AC power from an AC power supply 20 inthe welder 2. The transformer secondary winding XM is connected to theSCR network, by which the selective gating of the SCRs 8 or 10 connectsthe transformer secondary winding XM across the electrode 14 and theworkpiece 16 according to SCR gating signals at the gate terminalsthereof, in order to provide a first current to the welding circuit. TheSCR gate control signals GPOUT(+) and GPOUT(−) are derived in the welder2 from a phase angle firing control circuit 22, which provides gatepulse input signals GPIN(+) and GPIN(−) to a control circuit 24, whichin turn, provides the SCR gating signals GPOUT(+) and GPOUT(−) to theSCR pairs 8 and 10, respectively. For instance, in a portion of a firsthalf-cycle of the AC input power source (e.g., wherein a positivevoltage appears at the positive terminal of the secondary winding XM),the SCR gating signal GPOUT(+) is asserted by the control circuit 24 toplace SCRs 8 a and 8 b in a conductive state. Conversely, in a secondinput half-cycle, the SCR gating signal GPOUT(−) is asserted by thecontrol circuit 24 to place SCRs 10 a and 10 b in a conductive state.

Thus, as connected in FIG. 1, the first power supply 4 provides currentin the first half-cycle from the positive terminal of the secondary XMalong a current path through SCR 8 b, a jumper C-A, a welder outputchoke 26, a shunt 28, SCR 8 a, a jumper D-E, and a high frequency arcstarting generator 27 with a circuit secondary 30, to the electrode 14,and returning through the workpiece 16 and a jumper B-F. The choke 26 isdesigned for arc current stabilization during welding wherein thevoltage applied by the welder 2 across the output of the SCR network isbeing switched. The shunt 28 may have a resistance associated therewith,and may be used to sense the current flowing to the welding process, soas to provide feedback as to the actual output welding current of thewelder 2, for example, wherein such a feedback signal (e.g., a voltagemeasured across the terminals of the shunt 28) is supplied to the phaseangle firing control circuit 22 for comparison with the setpoint currentvalue I_(SET).

In the second half-cycle, the second SCRs 10 are gated to a conductingstate by the second SCR gating signal GPOUT(−), while the first SCRs 8are turned off by virtue of the current reversal at the secondary XM atthe end of the first half-cycle of the power supply 20. In this case,the current leaves the lower terminal of the secondary winding XM, andconducts through jumper B-F, the workpiece 16, electrode 14, highfrequency secondary winding 30 of generator 27, jumper D-E, SCR 10 a,jumper C-A, choke 26, shunt 28, and through the SCR 10 b into the upperterminal of the secondary winding XM. The phase angle firing controlcircuit 22 thus provides the signals GPIN(+) and GPIN(−) to the controlcircuit 24 having a controlled firing angle in the respective inputpower half-cycles prior to the end thereof in a manner well known in theart. In this manner, the signals GPIN(+) and GPIN(−) are asserted by thephase angle circuit 22 a controlled time period (e.g., corresponding toa controlled electrical phase angle) prior to the end of the positiveand negative half-cycles, in accordance with the desired setpointcurrent value. As is known, the SCRs 8, 10 will become conductive uponreceipt of the gating signal GPOUT from the control circuit 24 andremain in the conductive state until the end of the half-cycle, whereatthe current through the secondary winding XM changes polarity, unlesslatched in the conductive state through operation of the background orsecond power supply 6, as described in greater detail hereinafter.

The second power supply 6 is also connected to the SCR network and isactuated by first and second background signals BG(+) and BG(−),respectively, from the control circuit 24, by which the SCR network isoperable to selectively connect the second power supply 6 to the weldingcircuit according to the SCR control signals GPIN(+) and GPIN(−) so asto provide a second current to the welding circuit. Thus, the backgroundsupply 6 provides filling or background current to the welding circuitthrough the first power supply 4. The second power supply 6 comprisesfirst and second background DC power supplies 40 and 42, each havingpositive and negative output terminals, with background secondarywindings XB+ and XB−, and control power secondary windings XD+ and XD−,respectively, which are described in greater detail hereinafter withrespect to FIG. 11. The first background power supply 40 is connected tothe SCR network, which selectively connects the supply 40 to the weldingcircuit according to the signal GPOUT(+), and the second backgroundpower supply 42 is connected to the SCR network, which selectivelyconnects the supply 42 to the welding circuit according to the signalGPOUT(−), so as to provide the second current to the welding circuitthrough the SCR network (e.g., SCRs 8 and 10) of the first power supply4.

Referring also to FIG. 11, one implementation of the first backgroundpower supply 40 is illustrated in accordance with the present invention.As illustrated, the first background power supply 40 comprises a firstbackground DC power supply comprising a rectifier 44 receiving AC powerat about 63 vac from the secondary winding XB+, which is rectified to aDC voltage across a resistor 46 and a capacitor 48 of about 88 vdc. Thepower supply 40 operates to receive the signal BG(+) from the controlcircuit 24, which turns on the output transistor of an optical couplerdevice 50. The supply 40 further comprises another secondary winding XD+receiving AC power at about 18 vac from the AC power supply 20, which isrectified to about 24 vdc to provide power for the optical coupler 50and a first background switch 52, which is a field effect transistor(FET) device in the exemplary background power supply 40. The switch 52operates in response to the BG(+) signal from the control circuit 24(e.g., via the optical coupler 50) to provide the DC voltage across theresistor and capacitor 46, 48 to the SCR network through positive andnegative output terminals 54 and 56, respectively when the switch 52conducts. In the exemplary welder 2 of FIGS. 1 and 3, the first andsecond background power supplies 40 and 42 are constructed in similarfashion to one another as illustrated in FIG. 11. However, it will beappreciated that other background supplies may be constructed inaccordance with the present invention, apart from the exemplary supply40 of FIGS. 1, 3, and 11.

Referring also to FIGS. 5 and 6, further details of the exemplarycontrol circuit 24 are illustrated and described hereinafter in thecontext of AC mode operation. In operation in accordance with thepresent invention, the control circuit controls the actuation of theSCRS 8 and 10 by generating SCR gating signals GPOUT(+) and GPOUT(−)according to phase angle firing control signals GPIN (+) and GPIN(−) anda disable signal 60, where the disable signal is generated in thecontrol circuit 24 according to the mode of the welder as represented bya MODE_(IN) signal, the setpoint current value I_(SET), and an outputenable signal OE. The first SCR gating signal GPOUT(+) is connected tothe gate terminals of the first SCRs 8 a and 8 b in the power supply 4,and the second SCR gating signal GPOUT(−) is connected to the gateterminals of the second SCRs 10 a and 10 b. In addition to the SCRgating output signals, the control circuit 24 selectively provides thefirst and second background control signals BG(+) and BG(−) to the firstand second background power supplies 40 and 42, respectively, so as toprovide background current to the welding circuit in a controlledfashion according to the phase angle firing control signals GPIN (+) andGPIN(−) and the disable signal 60.

As shown in FIG. 6, the current setpoint value I_(SET) is increased froma minimum value and the output enable signal OE is asserted at time T1,which enables the first background control signal BG(+), and the SCRgating signal GPOUT(+) is asserted concurrently with the first SCRcontrol signal GPIN(+), while the second SCR gating signal GPOUT(−) isdisabled until the current setpoint value I_(SET) becomes greater than afirst reference value I_(AC) at time T2. At that point, the disablesignal 60 is released by transistor switch Q3 in FIG. 5 (e.g., goeshigh), whereby the next SCR control signal GPIN(−) causes acorresponding second SCR gating signal GPOUT(−) during a second powerhalf-cycle. Also, following the enabling of the second SCR gating signalat T2, the second background control signal BG(−) is enabled, allowingthe second background supply 42 to operate following the GPOUT(−)pulses, which disables background control signal BG(+). This operationcontinues with alternating assertion of the signals GPOUT(+) andGPOUT(−) (e.g., and the corresponding assertion of background controlsignals BG(+) and BG(−)) until the setpoint current value I_(SET) isbrought below I_(AC) at time T3.

As illustrated in FIG. 5, the control circuit 24 comprises a logiccircuit 24 a and a comparator circuit 24 b, wherein the logic circuit 24a is operatively connected to control the switching of the first andsecond background power supplies 40 and 42 and the SCR network, whilethe comparator circuit 24 b provides the disable signal 60 according tothe setpoint current value I_(SET) In particular, the comparator circuit24 b asserts the disable signal 60 as a low voltage by actuating thetransistor Q3 when the welder mode is AC (e.g., where the signalMODE_(IN) has a value of about 3 vdc in AC mode and about 0 volts in DCmode), and the setpoint current value I_(SET) is less than a firstreference value I_(AC). In the illustrated implementation, I_(AC) is setto about 0.15 vdc via a resistor divider network 62, corresponding to 9amps output welding current in the welder 2, although any appropriatevalue may be selected for the reference current value I_(AC.) Thecomparator circuit 24 b includes a quad comparator device 63 comprisinga first comparator 63 a, which asserts the transistor switch Q3 when thesetpoint current value I_(SET) is less than I_(AC), as well as a secondcomparator 63 b asserting Q3 when the mode input signal MODE_(IN)indicates AC mode operation for the welder 2. A third comparator 63 cprovides a MODE_(OUT) output signal, which is high when I_(SET) isgreater than a second reference value I_(HF) which can be used ifdesired to actuate continuous high frequency for AC mode at a higherI_(SET) level, and low otherwise for start only high frequency, and afourth comparator 63 d provides an output enable signal 65 operable whenhigh, to allow assertion of the first and second background controlsignals BG(+) and BG(−) by the logic circuit 24 a via gates 70 a and 70b, respectively, in accordance with an output enable input signal OE.The comparator circuit 24 b also includes a jumper 64 for selectingDC(−) or DC(+) operation when I_(SET) is below I_(AC). For instance,when the jumper 64 is set to DC(−) operation, the disable signal 60operates (e.g., when asserted by the transistor switch Q3) toselectively disable the first SCR gating signal GPOUT(+) and the firstbackground control signal BG(+). Alternative, where the jumper 64 is setto DC(+) operation, the disable signal 60 operates to selectivelydisable the second SCR gating signal GPOUT(−) and the second backgroundcontrol signal BG(−).

The logic circuit 24 a receives the disable signal 60 through theselected terminals of jumper 64, and if signal 60 is asserted (e.g.,low), the control circuit 24 disables the appropriate pair of signals byoperation of first and second NAND gates 66 a and 66 b in a first logiccircuit 68, which in turn, generate the first and second SCR gatingsignals GPOUT(+) and GPOUT(−). Thus, when disabled by signal 60, theNAND gates 66 provide a high output such that no SCR gating occurs. Thegating signals GPOUT(+) and GPOUT(−) also drive set and reset inputs toan RS flip-flop comprising first and second gates 70 a and 70 b in asecond logic circuit 72, where first and second output gates 74 a and 74b provide the background control signals BG(+) and BG(−) according tothe most recently asserted SCR gating signal (e.g., either GPOUT(+) orGPOUT(−)). In accordance with an aspect of the invention, the logiccircuit 24 a thus provides for connecting one of the background supplies40 or 42 to the welding circuit based on the corresponding SCR phaseangle firing control signal GPIN(+) or GPIN(−), and thereafter until theother such signal is asserted, or disabled by signal 65.

It is noted in this regard, that where the disable signal 60 causes oneof the SCR gating signals GPOUT(+) or GPOUT(−) to be disabled (e.g., bysetting the setpoint current value I_(SET) to less than about 9 amps inAC mode), one of the background supplies 40 or 42 will continue toprovide current to the welding circuit by latching the most recentlygated SCR 8 a or 10 a, thereby providing a conductive path forbackground current to continue to flow. Otherwise, in AC mode, thecontrol circuit 24 operates to gate the SCRs 8 or 10 to provide currentfrom the main supply secondary winding XM and the selected backgroundsupply 40 or 42 for a portion of each half-cycle, and thereafter tocontinue to supply background current until the next SCR gating pulsesignal GPOUT. Thus, as illustrated in the AC mode configuration of FIG.1, in the positive half-cycle, the SCRs 8 a and 8 b are turned on by apulse GPOUT(+) from the control circuit 24 and the phase angle controlcircuit 22, by which current from the upper terminal of the mainsecondary winding XM flows through SCRs 8 b and 8 a. At the same time,the pulse GPOUT(+) latches the flip-flop in the second logic circuit 72(FIG. 5) to provide the first background control signal BG(+) whichcauses background current to flow from the positive terminal thereof,through jumper C-A, choke 26, shunt 28, SCR 8 a, jumper D-E, highfrequency winding 30 of generator 27, and to the electrode 14 andworkpiece 16, returning through jumper B-F and the jumpered parallelcombination of two DC mode background resistors 80 a and 80 b and one oftwo AC mode resistors 82 a and 82 b.

Once the SCR gating pulse signal GPOUT(+) ends (e.g., causing the firstSCR 8 b to turn off after the polarity change of the main secondarywinding XM), the current from the first background supply 40 continuesto conduct through the above path, thereby latching the other first SCR8 a in the conductive state. This is because the flip-flop 70 a and 70 bin the control circuit 24 maintains the BG(+) control signal after thephase angle firing signal GPIN(+) and the corresponding gating signalGPOUT(+) are no longer asserted. Thus, the background current fromsupply 40 (e.g., set to about 5 amps in AC mode by operation of theresistor 82 a) conducts for an entire half-cycle. As can be seen fromFIGS. 1, 5, and 11, a similar situation occurs for the negativehalf-cycle, wherein the SCRs 10 a and 10 b are initially gated on by thesignal GPOUT(−), and thereafter, the control circuit 24 maintains thesecond background supply 42 to latch the SCR 10 a in the conductivestate, by which background current is supplied through resistor 82 b inthe absence of current from the first power supply 4.

Referring also to FIGS. 2A-2C, voltage and current curves areillustrated for the welder 2 in AC mode at various operating currents.In FIG. 2A, the curves 100 and 102 illustrate a case where the currentsetpoint value I_(SET) is adjusted to about 5 amps, and the second orbackground power supply 6 is set to provide about 5 amps or less of ACcurrent to the welding circuit (e.g., by appropriate selection of theresistors 82 in the second supply 6. The voltage 104 at the mainsecondary winding XM is illustrated in the upper curve 100 for threeexemplary half-cycles thereof, including a first input half-cycle 104 a,a second input half-cycle 104 b, and a subsequent first input half-cycle104 a′. In accordance with the setpoint value I_(SET), the first SCRs 8are gated at time 106 by the SCR gating signal GPOUT(+) from the controlcircuit 24 from the time 106 until time 108, causing a small currentpulse 110 in the output current provided to the welding circuit. Also attime 106, the control circuit 24 provides the first background controlsignal BG(+) to enable the first background power supply 40 to supplybackground current of about 5 amps or less to the welding circuitthereafter until time 112. Thus, the background current flows after themain supply current has been discontinued, so as to fill in the timeperiods between SCR gating pulse signals GPOUT(+) and GPOUT(−). At time112, the second SCR gating signal GPOUT(−) is asserted until time 114,causing a negative current pulse 116 in the output current. Also at 112,the control circuit 24 asserts the second background control signalBG(−) to provide background current from the second background supply 42thereafter until the next assertion of GPOUT(+).

It is noted in FIG. 2A, that positive current is supplied to the weldingcircuit from time 106 until time 112, thus establishing a first outputhalf-cycle 118 a, and that negative current is supplied from time 112until a subsequent gating signal GPOUT(+) is provided at time 120, thusestablishing a second output half-cycle 118 b, wherein the outputhalf-cycles 118 a and 118 b lag the input half-cycles 104 a and 104 b,respectively by a phase angle 122 generally equal to 180 degrees minusthe portion of each input cycle in which the SCR gating signal GPOUT isasserted. In addition, the periods between times 106 and 108, andbetween times 112 and 114 (e.g., the time periods during which the SCRsare energized by the control circuit 24 according to the SCR controlsignals GPIN provided by the phase angle firing control circuit 22 untilthe end of the half cycle conduction), may, but need not be of equalduration. For instance, the positive SCR period between times 106 and108 may be shorter than that between times 112 and 114 in AC welding,where a higher (e.g., negative) current is desired for the penetrationoutput half-cycle than for the (positive) cleaning output half-cycle.

In FIG. 2B, another exemplary case is illustrated in curves 100′ and102′, in which the current setpoint value I_(SET) is adjusted to betweenabout 5 and 7 amps. The first SCRs 8 are gated at time 106′ (e.g.,slightly earlier than time 106 in the previous case of FIG. 2A) by theSCR gating signal GPOUT(+) from the control circuit 24 from the time106′ until time 108, causing a somewhat larger current pulse 110′ in theoutput current. Also at time 106′, the control circuit 24 provides thefirst background control signal BG(+) to enable the first backgroundpower supply 40 to supply background current at about 5 amps or less tothe welding circuit thereafter until time 112′ (e.g., earlier than time112 of FIG. 2A). At time 112′, the second SCR gating signal GPOUT(−) isasserted until time 114, causing a negative current pulse or pulse 116′in the output current 12. Also at 112′, the control circuit 24 assertsthe second background control signal BG(−) to provide background currentfrom the second background supply 42 thereafter until the next assertionof GPOUT(+). Positive current is thus supplied to the welding circuitfrom time 106′ until time 112′, thus establishing a first outputhalf-cycle 118 a, and negative current is supplied from time 112′ untila subsequent gating signal GPOUT(+) is provided at time 120′, thusestablishing a second output half-cycle 118 b, wherein the phase lagangle 122′ in FIG. 2B between output half-cycles 118 a and 118 b and theinput half-cycles 104 a and 104 b, respectively, is smaller than phaseangle 122 of FIG. 2A, due to the earlier firing angle of the SCR controlsignals GPIN (and the corresponding SCR gating signals GPOUT), resultingfrom the somewhat higher current setpoint value I_(SET).

Yet another case is illustrated in curves 100″ and 102″ of FIG. 2C,wherein the current setpoint value I_(SET) is set well above 7 amps.SCRs 8 are gated at time 106″ (e.g., earlier in the first inputhalf-cycle 104 a than time 106′ in FIG. 2B) by the SCR gating signalGPOUT(+) from the time 106″ until the time 108, causing a relativelylengthy current pulse 110″ in the output current 12. BG(+) is assertedat 106″, causing the background power supply 40 to provide backgroundcurrent to the welding circuit from time 106″ until time 112″. At time112″, the second SCR gating signal GPOUT(−) is asserted until time 114,causing a negative current pulse 116″ in the output current. Also at112″, BG(−) is asserted to provide background current from the secondbackground supply 42 thereafter until the next assertion of GPOUT(+) at120″. In the exemplary case of FIG. 2C, the phase lag angle 122″ isrelatively small compared with the phase lags 122 and 122′ of FIGS. 2Aand 2B, respectively, due to the higher current setpoint value I_(SET).

Referring briefly to FIGS. 7 and 8, welder output current curve 300 ofFIG. 7 corresponds to the operation of the main supply 4 and the firstbackground supply 40 in the first or positive output half-cycle, whereinthe portion 300 a includes current from the supply 4 as well asbackground current from the first background supply 40, and the portion300 b is background current only. A second curve 302 of FIG. 7illustrates operation of the main supply 4 and the second backgroundsupply 42 in the second or negative output half-cycle, wherein theportion 302 a includes current from the supply 4 as well as backgroundcurrent from the second background supply 42, and the second portion 302b is background current only. As shown in FIG. 8, the AC mode weldingcurrent 12 applied from the welder 2 to the welding circuit in FIG. 1 isthe combination of the curves 300 and 302 of FIG. 7.

Thus, as shown in FIGS. 2A-2C, 7, and 8, the welder 2 in AC mode can besuccessfully operated to provide very low AC welding current to thewelding circuit, wherein the problems associated with maintaining an arcat such low currents are avoided or mitigated by filling in the spacesbetween SCR gating pulses GPOUT with background current from thebackground supplies 40 and 42, conducted through the SCR network (e.g.,SCRs 8 a and 10 a) of the first power supply 4. In addition, in AC mode,the welder 2 may continue current flow (either positive DC current ornegative DC current) below about 7 amps (e.g., such as about 5 amps inthe illustrated implementation) when the AC current setpoint I_(SET) isbrought below 5 amps. Thus, in FIG. 5, where the jumper 64 is set to theDC(+) position, the control circuit 24 operates in AC mode to disableGPOUT(−) signals and BG(−) signals at such low current settings, andcontinues positive DC current to the welding circuit via the supply 40through the latched SCR 8 a. Similarly, if the jumper 64 is set to theDC(−) position, the control circuit 24 disables GPOUT(+) signals andBG(+) signals at such low current settings, and continues negative DCcurrent to the welding circuit via the supply 42 through the latched SCR10 a.

As discussed above, the exemplary welder 2 may be operated in DC as wellas AC modes, whereby a wide variety of welding operations may besupported, through simple switching of AC/DC mode switch 3. Referringnow to FIG. 3, the TIG welder 2 is connected for DC welding operation,via the illustrated jumper settings of the AC/DC mode switch 3 (e.g.,jumper settings C-F, D-B), by which a current path is provided during aportion of a positive input power half-cycle from the positive (e.g.,upper) terminal of the main power supply transformer secondary XM,through first SCR 8 b, jumper C-F, workpiece 16, electrode 14, highfrequency winding 30 of generator 27, jumper E-A, choke 26, shunt 28,and first SCR 8 a, to the lower terminal of transformer secondarywinding XM. In the DC mode connection of FIG. 3, moreover, a flybackmode diode 130 is connected in the SCR network across SCRs 8 a and 10.The portion of the first input half-cycle in which the SCRs 8 are gatedon is again determined by the length of the first SCR control signalGPIN(+) from the phase angle firing control circuit 22 (FIG. 1) and thecorresponding SCR gating signal GPOUT(+) (e.g., unless disabled by thecontrol circuit 24). During a portion of a negative input half-cycle inwhich the SCR gating signal GPOUT(−) is asserted, SCRs 8 are not gated,and second SCRs 10 are gated on, whereby the main supply current leavesthe lower terminal of secondary winding XM, conducting through thejumper B-D, SCR 10 a, jumper C-F, workpiece 16, electrode 14, highfrequency winding 30 of generator 27, jumper E-A, choke 26, shunt 28,and second SCR 10 b, to the upper terminal of transformer secondarywinding XM.

The control circuit 24 operates in the DC mode to also selectivelyenable one of the background DC supplies 40 and 42 as the correspondingSCR pairs 8 or 10 are gated, wherein the enabled background supply 40 or42 latches the selected SCR 8 a or 10 a, after the termination of thecorresponding SCR gating signal GPOUT(+) or GPOUT(−), respectively. Inthe positive input half-cycle (e.g., where the voltage across the topand bottom terminals of the secondary winding XM is positive), the firstSCR gating signal GPOUT(+) is asserted to turn on SCRs 8 during aportion thereof, and the first background control signal BG(+) isasserted, whereby background current from the first background supply 40leaves the plus terminal thereof, and conducts through jumper C-F,workpiece 16, electrode 14, high frequency winding 30, jumper E-A, choke26, shunt 28, first SCR 8 a, the parallel combination of DC moderesistors 80 a and 80 b, and the resistor 82 a to the negative terminalof the first background supply 40. This background current from thesupply 40 latches the SCR 8 a on after removal of the SCR gating signalGPOUT(+) thereto, such that background current continues to flowtherethrough to the welding cycle even after current from the mainsecondary winding XM is discontinued by SCR operation turning off SCR 8b.

Conversely, in the negative input half-cycle, the second SCR gatingsignal GPOUT(−) is asserted to turn on SCRs 10, and the first backgroundcontrol signal BG(−) is asserted, whereby background current from thesecond background supply 42 leaves the plus terminal thereof, andconducts through resistor 82 b, parallel resistors 80 a and 80 b, jumperB-D, SCR 10 a, jumper C-F, workpiece 16, electrode 14, high frequencywinding 30, jumper E-A, choke 26, shunt 28, and returns to the negativeterminal of the supply 42, wherein the background current from supply 42operates to latch the second SCR 10 a in the on or conductive state evenafter discontinuance of the SCR gating signal GPOUT(−) by the controlcircuit 24.

It is noted that the values of the DC and AC mode resistors 80 and 82are set to about 12.5 OHMs and 7.5 OHMs, respectively, in theillustrated implementation, by which the background current supplied bythe background supplies 40 and 42 is about 2 amps in DC operation.However, it will be appreciated that other background DC or AC modecurrent levels may be achieved in accordance with the present invention,for example, by selecting different values for the resistors 80 and/or82 or other levels of background voltage supplies 40 and 42. Thus, theinvention advantageously provides for very low current DC weldingoperation (e.g., below about 5 amps, such as about 2 amps DC), as wellas for very low AC current welding operation (e.g., about 7 amps AC orless) as discussed above.

Referring also to FIGS. 4A-4C, the welder 2 may operate to provide DCcurrent, either positive or negative, to the welding circuit with orwithout SCR gating signals GPOUT, depending upon the current setpointvalue I_(SET), such that very low current operation is facilitated(e.g., about 2 amps DC). As illustrated in FIG. 3, the DC mode jumpersettings provide for background current of a single polarity to bedelivered to the welding circuit regardless of which of the backgroundsupplies 40 or 42 is activated at any given time. Thus, where DC(+) orDC(−) welding is selected by the mode switch 3, welding at extremely lowcurrent setting is achieved by either the positive background supply 40without repeated SCR gating signals GPOUT through the latching of SCR 8a in the on or conductive state by the background current from thesupply 40, or the negative background supply 42 without repeated SCRgating signals GPOUT through the latching of SCR 10 a by the backgroundcurrent from the supply 42. In other words, the polarity switch 3selects the DC polarity, but depending on which SCR (8 a or 10 a) waslast conducting the starting pulse, the minimum output current could besupplied by either background supply 40 or 42.

DC welding without SCR gating pulses (e.g., other than an initialstarting pulse during arc initiation) is illustrated in FIG. 4A, whereinvoltage and current curves 200 and 202 are illustrated for the welder 2in DC mode for a setpoint value I_(SET) of about 2 amps. In FIG. 4A, onecase is illustrated where the current setpoint value I_(SET) is adjustedto about 2 amps, and the background power supply 6 is set to provideabout 2 amps or less by the values of the resistors 80 and 82. Thevoltage 204 at the main secondary winding XM is illustrated in the uppercurve 200 for three exemplary half-cycles thereof, including a firstinput half-cycle 204 a, a second input half-cycle 204 b, and asubsequent first input half-cycle 204 a′. The first SCR 8 a, having beenpreviously gated on for a weld start by a gating pulse GPOUT(+), isthereafter maintained or latched in the conductive state by backgroundcurrent 205 provided to the welding circuit by the first backgroundsupply 40.

In FIG. 4B, voltage and current curves 200′ and 202′ are illustratedwhere the setpoint value I_(SET) has been increased slightly above 2amps. In this case, the control circuit 24 provides GPOUT(+) pulses attime 206 until time 208, and again at time 220 and an intervening pulseGPOUT(−) at time 212 until time 214. The gating of the first SCRs 8 aand 8 b via the pulse GPOUT(+) causes a positive current pulse 210 inthe welding current 205, which is a composite current comprising mainand background supply currents from the supplies 4 and 6, respectively.Also at time 206, the control circuit 24 provides the first backgroundcontrol signal BG(+) to enable the first background power supply 40 tosupply background current of about 2 amps thereafter until time 212.Thus, the background current flows after the main supply current hasbeen discontinued, so as to fill in the time periods between SCR gatingpulse signals GPOUT(+) and GPOUT(−). At time 212, the second SCR gatingsignal GPOUT(−) is asserted until time 214, causing another positivecurrent pulse 210 in the output current 205. Also at 212, the controlcircuit 24 asserts the second background control signal BG(−) to providebackground current from the second background supply 42 thereafter untilthe next assertion of GPOUT(+) at 220. In FIG. 4C, voltage and currentcurves 200″ and 202″ illustrate the current setpoint value I_(SET) beingincreased even further above 2 amps in the illustrated example, whereinthe SCR gating signals GPOUT are longer than those of FIG. 4B, resultingin earlier gating times 206′, 212′, 220′, and higher current pulses orpeaks 210′ in the output current 205′.

Referring also to FIGS. 9 and 10, welder output current curve 400 ofFIG. 9 illustrates the operation of the main supply 4 and the secondpower supply 6 in the DC operating mode, wherein the portion 400 aincludes welding current 12 from the supply 4 as well as backgroundcurrent from the first background supply 40 and a second portion 400 bcomprises background current only, supplied by the first backgroundsupply 40. Thereafter, a third portion 400 c includes current from themain supply 4 and the second background power supply 42 and a fourthportion 400 d comprises current solely from the second background supply42, after which the pattern is repeated as the first background supply40 is again enabled by the control circuit 24. In FIG. 10, a very lowcurrent operation is illustrated for DC welding mode, wherein thesetpoint current value I_(SET) has been decreased to about 2 amps,similar to the curves of FIG. 4A described above. Following an initialSCR gating pulse GPOUT(+) (not shown), the first (positive) backgroundsupply 40 provides current 12 to the welding circuit, thereby latchingthe first SCR 8 a in the conductive state, and thereafter provides lowlevel current of about 2 amps DC in regions 402 a, 402 b, and 402 c.

Referring now to FIGS. 1 and 12, the welder 2 may be used to implementimproved arc starting techniques in accordance with other aspects of theinvention. For example, in the DC operating mode, the welder 2 may bestarted using the techniques illustrated and described in Samodell U.S.Pat. No. 6,388,232. In AC mode, the welder allows a choice ofconventional starting techniques, as well as DC starting, with atransition into AC welding operation following arc initiation. FIG. 12illustrates the welder output voltage VS related to the main supply 4,as well as the welding circuit current 12 in three phases of a novel arcstarting methodology 500, 510, and 520, respectively in accordance withthe present invention, wherein the control circuit 24 is configured withjumper 64 (FIG. 5) in the DC(+) position.

The control circuit 24 provides only gating signal GPOUT(+) for a startpulse duration in the first phase 500 for a portion 502 of a first inputpower half-cycle while the high frequency generator 27 of FIG. 1energizes the high frequency winding 30 in the welding circuit. Thisconnects the transformer secondary winding XM to the welding circuit viathe SCRs 8 a and 8 b to provide relatively high positive polaritycurrent 12 to the welding circuit during the start pulse period 502. Thecontrol circuit 24 also energizes the first background supply 40 duringthe period 502, thus providing background level current 12 to thewelding circuit as described above. Alternatively, the second backgroundsupply 42 may be employed with the SCRs 10 a and 10 b, for example,where the jumper 64 of the control circuit 24 is set to the DC(−)position.

Following the starting pulse at 502, the background supply 40 provideslow level DC current (e.g., about 5 amps) with the SCR 8 a latchedthereby, as illustrated at 504 in FIG. 12. This operation continuesuntil the setpoint current value I_(SET) is increased to a point (e.g.,between about 5 and 7 amps) where the control circuit 24 again beginsgating the first SCRs 8 a and 8 b at times 512 in the second phase 510,causing corresponding positive current pulses or peaks 514. Furtherincreasing the current setpoint value I_(SET) above about 7 amps inphase 520 operates to turn off switch Q3 of FIG. 5 (e.g., throughtransition in the output of comparator 63 a), whereby the controlcircuit 24 operates to alternatively assert the SCR gating pulsesGPOUT(+) and GPOUT(−), thus alternatively firing the SCRs 8 and 10(e.g., and also alternatively enabling the first and second backgroundsupplies 40 and 42 via background control signals BG(+) and BG(−)),causing the welder 2 to supply alternating polarity AC current to thewelding circuit. Once the AC operation has been achieved in phase 520,moreover, high frequency arc stabilization techniques may be employed,for example, using the high frequency generator 27 and the associatedwinding 30 (FIG. 1). Thus, the invention provides for selection of arcstarting methodologies, and also facilitates starting techniquesheretofore not achievable in AC welding operations. This may beadvantageously employed, for example, in association with AC aluminumwelding applications, wherein the inventor has found that the DCstarting technique described above provides optimized low currentstarting and welding stability, with faster heating of the tungstenelectrode 14, and improved cleaning during the positive outputhalf-cycle for thin workpieces 16, without the high frequency arc“dancing” and workpiece pitting conditions found in prior welders at lowcurrent settings.

The illustrated welding system 2 and the controls and power suppliesthereof are further applicable to various applications or situationsbeyond starting in which stable background current operation aredesired. FIG. 13 illustrates one such application in performing a craterweld at the end of a TIG welding cycle or operation in a TIG weldingsystem 600. In this example, the above described AC/DC TIG welder 2 isused to provide welding current to the welding operation in accordancewith the present invention, wherein a workpiece 602 is welded to createa weld 604 by controlled deposition of molten metal 606 from a fillerwire 608. A TIG torch 610 housing a tungsten electrode 618 is energizedby an electrical contact 614 as generally illustrated and describedabove, and a nozzle 612 surrounds the contact 614 and electrode 618 toprovide a gas chamber or passage through which shielding gas is providedto shield a welding arc A and the welding process. As seen in FIG. 13,at the end of the weld, immediate stoppage of the welding arc A mayresult in formation of an undesirable crater due to abrupt terminationof the large arc force commensurate with high current welding operation.Accordingly, the principles of the present invention may be used toslowly drop the welding current level from the high normal level (asshown in phase 520 of FIG. 12) to an intermediate level (e.g., betweenabout 5 and 7 A as shown in phase 510 of FIG. 12), and thereafter to abackground current level at or below about 5 A (phase 500 of FIG. 12) tofill in the crater. Thus, the same circuitry described above thatswitches the low current from DC to AC during starting can be employedto switch back to low level DC as the current set point is reduced forcrater-filling at the end of the weld. This advantageously facilitatesprecise filling of the weld crater with stable low-level DC currentwithout the adverse effects of “dancing” high frequency in the arccommonly experienced in AC crater filling.

In accordance with another aspect of the invention, the backgroundsupply levels can be different in the positive and negative polarities,whereby the background current need not be centered around zero. Thiscan be implemented in any suitable manner, for instance, by providingdifferent values of current controlling resistors 82 a and 82 b in thewelder 2 (e.g., FIGS. 1 and 5 above), and/or by providing differentamplitude supplies 40 and 42. In this aspect of the invention, thebackground current imbalance can be set for optimizing cleaning and/orpenetration, wherein more negative bias provides higher weld penetrationand more positive background bias improves cleaning and better arcstability to inhibit or combat the loss of positive half-wave current orAC TIG “rectification” often found at higher weld currents.

While the invention has been illustrated and described hereinabove withrespect to one or more implementations, equivalent alterations andmodifications will occur to others skilled in the art upon reading andunderstanding this specification and the annexed drawings. In particularregard to the various functions performed by the above describedcomponents (assemblies, devices, systems, circuits, and the like), theterms (including a reference to a “means”) used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of theinvention. In addition, although a particular feature of the inventionmay have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular application. Furthermore, to the extent thatthe terms “including”, “includes”, “having”, “has”, “with”, or variantsthereof are used in the detailed description and/or in the claims, suchterms are intended to be inclusive in a manner similar to the term“comprising.”

Having thus described the invention, the following is claimed:
 1. A TIGwelder operable to provide an output welding current through a gap in awelding circuit between an electrode and a workpiece according to asetpoint current value, said welder comprising: a first power supplycomprising a first transformer secondary winding; an SCR networkconnected to said first transformer secondary winding, said SCR networkbeing operable to selectively connect said first transformer secondarywinding to said welding circuit according to SCR control signals so asto provide a first current to said welding circuit; a second powersupply connected to said SCR network, said SCR network being operable toselectively connect said second power supply to said welding circuitaccording to said SCR control signals so as to provide a second currentto said welding circuit through at least one SCR in said SCR network ofsaid first power supply; a control circuit connected to said SCR networkand to said second power supply and operable to selectively connect saidSCR control signals to said SCR network according to said setpointcurrent value; and an AC/DC switch with a first switch condition and asecond opposite switch condition; wherein said welder is operable in anAC mode to provide an AC output welding current through said gap in saidwelding circuit according to said setpoint current value when said AC/DCswitch is in said first switch condition, and is operable in a DC modeto provide a DC output welding current through said gap in said weldingcircuit according to said setpoint current value when said AC/DC switchis in said second switch condition; wherein said SCR network is operablein said AC mode to provide current of a first polarity to said weldingcircuit from at least one of said first and second power supplies in afirst half-cycle, and to provide current of a second polarity to saidwelding circuit from at least one of said first and second powersupplies in a second half-cycle according to said setpoint currentvalue; and wherein said SCR network is operable in said DC mode toprovide current of one of said first and second polarities to saidwelding circuit from at least one of said first and second powersupplies according to said setpoint current value.
 2. The welder ofclaim 1: wherein said SCR network comprises: a first SCR operating inone of a conductive state and a nonconductive state according to a firstSCR control signal from said control circuit, and a second SCR operatingin one of a conductive state and a nonconductive state according to asecond SCR control signal from said control circuit; and wherein saidsecond power supply comprises: a first background power supply connectedto said SCR network, said first SCR being operable to selectivelyconnect said first background power supply to said welding circuitaccording to said first SCR control signal so as to provide said secondcurrent to said welding circuit, and a second background power supplyconnected to said SCR network, said second SCR being operable toselectively connect said second background power supply to said weldingcircuit according to said second SCR control signal so as to providesaid second current to said welding circuit.
 3. The welder of claim 2:wherein said first background power supply comprises: a first backgroundDC power supply, and a first background switch connected to said firstbackground DC power supply, said first background switch being operableto connect said first background DC power supply to said SCR networkaccording to a first background control signal; wherein said secondbackground power supply comprises: a second background DC power supply,and a second background switch connected to said second background DCpower supply, said second background switch being operable to connectsaid second background DC power supply to said SCR network according toa second background control signal; and wherein said control circuitcomprises: a logic circuit connected to said first and second backgroundpower supplies and said SCR network, said logic circuit being operableto selectively provide said first and second background control signalsaccording to first and second SCR control signals, respectively, andaccording to a disable signal, and to selectively provide first andsecond SCR gating signals to said first and second SCRs according tosaid first and second SCR control signals, respectively, and accordingto said disable signal, and a comparator circuit connected to said logiccircuit and operable to provide said disable signal according to saidsetpoint current value.
 4. The welder of claim 3, wherein said first SCRcontrol signal is active during a portion of a first half-cycle where avoltage associated with said first transformer secondary is a firstpolarity and said second SCR control signal is active during a portionof a second half-cycle where a voltage associated with said firsttransformer secondary is a second polarity.
 5. The welder of claim 4:wherein said logic circuit is operable to selectively assert said firstbackground control signal during said portion of said first half-cycleand thereafter until said second SCR control signal is active, and toassert said second background control signal during said portion of saidsecond half-cycle and thereafter until said first SCR control signal isactive: and wherein said logic circuit is operable to assert said firstSCR gating signal to turn on said first SCR during said portion of saidfirst half-cycle and to assert said second SCR gating signal to turn onsaid second SCR during said portion of said first half-cycle such thatsaid second power supply latches said first SCR in said conductive stateafter said first half-cycle until said second background control signalis asserted and latches said second SCR in said conductive state aftersaid second half-cycle until said first background control signal isasserted.
 6. The welder of claim 3, wherein said logic circuit isoperable to selectively refrain from providing said first SCR gatingsignal and said first background control signal or to selectivelyrefrain from providing said second SCR gating signal and said secondbackground control signal according to said disable signal.
 7. Thewelder of claim 3, wherein said comparator circuit is operable tocompare said setpoint current value with a first given current value,and wherein said logic circuit is operable to selectively refrain fromproviding one of said first and second SCR gating signals if saidsetpoint current value is less than said first given value, and toselectively refrain from providing one of said first and secondbackground control signals if said setpoint current value is less thansaid first given value.
 8. The welder of claim 5, wherein said logiccircuit is operable to selectively refrain from providing said first SCRgating signal and said first background control signal or to selectivelyrefrain from providing said second SCR gating signal and said secondbackground control signal according to said disable signal.
 9. Thewelder of claim 8, wherein said comparator circuit is operable to assertsaid disable signal in said AC mode if said setpoint current value isless than a first given value.
 10. The welder of claim 9, wherein saidfirst given value is about 7 amps.
 11. The welder of claim 5, whereinsaid comparator circuit is operable to compare said setpoint currentvalue with a first given current value, and wherein said logic circuitis operable to selectively refrain from providing one of said first andsecond SCR gating signals in said AC mode if said setpoint current valueis less than said first given value, and to selectively refrain fromproviding one of said first and second background control signals insaid AC mode if said setpoint current value is less than said firstgiven value.
 12. The welder of claim 5, wherein said comparator circuitis operable to compare said setpoint current value with a first givencurrent value, and wherein said logic circuit is operable to selectivelyrefrain from providing said first SCR gating signal and said firstbackground control signal if said setpoint current value is less thansaid first given value in said AC mode.
 13. The welder of claim 5,wherein said comparator circuit is operable to compare said setpointcurrent value with a first given current value, and wherein said logiccircuit is operable to selectively refrain from providing said secondSCR gating signal and said second background control signal if saidsetpoint current value is less than said first given value in said ACmode.
 14. The welder of claim 11, wherein said logic circuit comprises aflip-flop operable to selectively assert said first background controlsignal during said portion of said first half-cycle according to saidfirst SCR control signal and thereafter until said second SCR controlsignal is active, and to assert said second background control signalduring said portion of said second half-cycle according to said secondSCR control signal and thereafter until said first SCR control signal isactive.
 15. The welder of claim 14, wherein said flip-flop is an RSflip-flop comprising: a set input controlled according to said first SCRcontrol signal, a reset input controlled according to said second SCRcontrol signal, a first output operable to selectively assert said firstbackground control signal if said flip-flop is set, and a second outputoperable to selectively assert said second background control signal ifsaid flip-flop is reset.
 16. The welder of claim 1: wherein said SCRnetwork comprises: a first SCR operating in one of a conductive stateand a nonconductive state according to a first SCR control signal fromsaid control circuit, and a second SCR operating in one of a conductivestate and a nonconductive state according to a second SCR control signalfrom said control circuit; and wherein said second power supplycomprises: a first background power supply connected to said SCRnetwork, said first SCR being operable to selectively connect said firstbackground power supply to said welding circuit according to said firstSCR control signal so as to provide said second current to said weldingcircuit, and a second background power supply connected to said SCRnetwork, said second SCR being operable to selectively connect saidsecond background power supply to said welding circuit according to saidsecond SCR control signal so as to provide said second current to saidwelding circuit.
 17. The welder of claim 16: wherein said firstbackground power supply comprises: a first background DC power supply,and a first background switch connected to said first background DCpower supply, said first background switch being operable to connectsaid first background DC power supply to said SCR network according to afirst background control signal; wherein said second background powersupply comprises: a second background DC power supply, and a secondbackground switch connected to said second background DC power supply,said second background switch being operable to connect said secondbackground DC power supply to said SCR network according to a secondbackground control signal; and wherein said control circuit comprises: alogic circuit connected to said first and second background powersupplies and said SCR network, said logic circuit being operable toselectively provide said first and second background control signalsaccording to first and second SCR control signals, respectively, andaccording to a disable signal, and to selectively provide first andsecond SCR gating signals to said first and second SCRs according tosaid first and second SCR control signals, respectively, and accordingto said disable signal, and a comparator circuit connected to said logiccircuit and operable to provide said disable signal according to saidsetpoint current value.
 18. The welder of claim 17, wherein said logiccircuit is operable to selectively refrain from providing said first SCRgating signal and said first background control signal or to selectivelyrefrain from providing said second SCR gating signal and said secondbackground control signal according to said disable signal.
 19. Thewelder of claim 17, wherein said comparator circuit is operable tocompare said setpoint current value with a first given current value,and to selectively refrain from providing one of said first and secondSCR gating signals if said setpoint current value is less than saidfirst given value, and to selectively refrain from providing one of saidfirst and second background control signals if said setpoint currentvalue is less than said first given value.
 20. The welder of claim 17,wherein said first SCR control signal is active during a portion of afirst half-cycle where a voltage associated with said first transformersecondary is a first polarity and said second SCR control signal isactive during a portion of a second half-cycle where a voltageassociated with said first transformer secondary is a second polarity.21. The welder of claim 20: wherein said logic circuit is operable toselectively assert said first background control signal during saidportion of said first half-cycle and thereafter until said second SCRcontrol signal is active, and to assert said second background controlsignal during said portion of said second half-cycle and thereafteruntil said first SCR control signal is active; and wherein said logiccircuit is operable to assert said first SCR gating signal to turn onsaid first SCR during said portion of said first half-cycle and toassert said second SCR gating signal to turn on said second SCR duringsaid portion of said first half-cycle such that said second power supplylatches said first SCR in said conductive state after said firsthalf-cycle until said second background control signal is asserted andlatches said second SCR in said conductive state after said secondhalf-cycle until said first background control signal is asserted. 22.The welder of claim 16, wherein said first SCR control signal is activeduring a portion of a first half-cycle where a voltage associated withsaid first transformer secondary is a first polarity and said second SCRcontrol signal is active during a portion of a second half-cycle where avoltage associated with said first transformer secondary is a secondpolarity.
 23. The welder of claim 22: wherein said logic circuit isoperable to selectively assert said first background control signalduring said portion of said first half-cycle and thereafter until saidsecond SCR control signal is active, and to assert said secondbackground control signal during said portion of said second half-cycleand thereafter until said first SCR control signal is active: andwherein said logic circuit is operable to assert said first SCR gatingsignal to turn on said first SCR during said portion of said firsthalf-cycle and to assert said second SCR gating signal to turn on saidsecond SCR during said portion of said first half-cycle such that saidsecond power supply latches said first SCR in said conductive stateafter said first half-cycle until said second background control signalis asserted and latches said second SCR in said conductive state aftersaid second half-cycle until said first background control signal isasserted.
 24. The welder of claim 23, wherein said logic circuitcomprises a flip-flop operable to selectively assert said firstbackground control signal during said portion of said first half-cycleaccording to said first SCR control signal and thereafter until saidsecond SCR control signal is active, and to assert said secondbackground control signal during said portion of said second half-cycleaccording to said second SCR control signal and thereafter until saidfirst SCR control signal is active.
 25. A TIG welder operable to providean output welding current through a gap in a welding circuit between anelectrode and a workpiece according to a setpoint current value, saidwelder comprising: first power supply means for supplying weldingcurrent comprising an SCR network connected to a first transformersecondary winding, said SCR network being operable to selectivelyconnect said first transformer secondary winding to said welding circuitaccording to SCR control signals to provide a first current to saidwelding circuit; second power supply means for supplying welding currentbeing connected to said SCR network, said SCR network being furtheroperable to selectively connect said second power supply means to saidwelding circuit through at least one SCR in said SCR network accordingto said SCR control signals to provide a second current to said weldingcircuit through said first power supply means; and control means forcontrolling said SCR network being connected to said SCR network andsaid second power supply means and operable to selectively connect saidSCR control signals to said SCR network according to said setpointcurrent value, wherein said SCR network comprises: a first SCR operatingin one of a conductive state and a nonconductive state according to afirst SCR control signal from said control means, and a second SCRoperating in one of a conductive state and a nonconductive stateaccording to a second SCR control signal from said control means; andwherein said second power supply means comprises: a first backgroundpower supply connected to said SCR network, said first SCR beingoperable to selectively connect said first background power supply tosaid welding circuit according to said first SCR control signal so as toprovide said second current to said welding circuit, and a secondbackground power supply connected to said SCR network, said second SCRbeing operable to selectively connect said second background powersupply to said welding circuit according to said second SCR controlsignal so as to provide said second current to said welding circuit. 26.The welder of claim 25: wherein said SCR network is operable in an ACmode to provide current of a first polarity to said welding circuit fromat least one of said first power supply means and said second powersupply means in a first half-cycle, and to provide current of a secondpolarity to said welding circuit from at least one of said first powersupply means and said second power supply means in a second half-cycleaccording to said setpoint current value: and wherein said SCR networkis operable in a DC mode to provide current of one of said first andsecond polarities to said welding circuit from at least one of saidfirst power supply means and said second power supply means according tosaid setpoint current value.
 27. The welder of claim 26: wherein saidfirst background power supply comprises: a first background DC powersupply, and a first background switch connected to said first backgroundDC power supply, said first background switch being operable to connectsaid first background DC power supply to said SCR network according to afirst background control signal; wherein said second background powersupply comprises: a second background DC power supply, and a secondbackground switch connected to said second background DC power supply,said second background switch being operable to connect said secondbackground DC power supply to said SCR network according to a secondbackground control signal; and wherein said control means comprises: alogic circuit connected to said first and second background powersupplies and said SCR network, said logic circuit being operable toselectively provide said first and second background control signalsaccording to first and second SCR control signals, respectively, andaccording to a disable signal, and to selectively provide first andsecond SCR gating signals to said first and second SCRs according tosaid first and second SCR control signals, respectively, and accordingto said disable signal, and a comparator circuit connected to said logiccircuit and operable to provide said disable signal according to saidsetpoint current value.
 28. The welder of claim 27, wherein said logiccircuit is operable to selectively refrain from providing said first SCRgating signal and said first background control signal or to selectivelyrefrain from providing said second SCR gating signal and said secondbackground control signal according to said disable signal.
 29. Thewelder of claim 27: wherein said comparator circuit is operable tocompare said setpoint current value with a first given current value;and wherein said logic circuit is operable to selectively refrain fromproviding one of said first and second SCR gating signals if saidsetpoint current value is less than said first given value, and toselectively refrain from providing one of said first and secondbackground control signals if said setpoint current value is less thansaid first given value.
 30. The welder of claim 27, wherein said firstSCR control signal is active during a portion of a first half-cyclewhere a voltage associated with said first transformer secondary is afirst polarity and said second SCR control signal is active during aportion of a second half-cycle where a voltage associated with saidfirst transformer secondary is a second polarity.
 31. The welder ofclaim 30: wherein said logic circuit is operable to selectively assertsaid first background control signal during said portion of said firsthalf-cycle and thereafter until said second SCR control signal isactive, and to assert said second background control signal during saidportion of said second half-cycle and thereafter until said first SCRcontrol signal is active; and wherein said logic circuit is operable toassert said first SCR gating signal to turn on said first SCR duringsaid portion of said first half-cycle and to assert said second SCRgating signal to turn on said second SCR during said portion of saidfirst half-cycle such that said second power supply latches said firstSCR in said conductive state after said first half-cycle until saidsecond background control signal is asserted and latches said second SCRin said conductive state after said second half-cycle until said firstbackground control signal is asserted.
 32. The welder of claim 31,wherein said logic circuit is operable to selectively refrain fromproviding said first SCR gating signal and said first background controlsignal or to selectively refrain from providing said second SCR gatingsignal and said second background control signal according to saiddisable signal.
 33. The welder of claim 32, wherein said comparatorcircuit is operable to assert said disable signal in said AC mode ifsaid setpoint current value is less than a first given value.
 34. Thewelder of claim 31: wherein said comparator circuit is operable tocompare said setpoint current value with a first given current value;and wherein said logic circuit is operable to selectively refrain fromproviding one of said first and second SCR gating signals in said ACmode if said setpoint current value is less than said first given value,and to selectively refrain from providing one of said first and secondbackground control signals in said AC mode if said setpoint currentvalue is less than said first given value.
 35. The welder of claim 31:wherein said comparator circuit is operable to compare said setpointcurrent value with a first given current value; and wherein said logiccircuit is operable to selectively refrain from providing said first SCRgating signal and said first background control signal if said setpointcurrent value is less than said first given value in said AC mode. 36.The welder of claim 31: wherein said comparator circuit is operable tocompare said setpoint current value with a first given current value;and wherein said logic circuit is operable to selectively refrain fromproviding said second SCR gating signal and said second backgroundcontrol signal if said setpoint current value is less than said firstgiven value in said AC mode.
 37. The welder of claim 34, wherein saidlogic circuit comprises a flip-flop operable to selectively assert saidfirst background control signal during said portion of said firsthalf-cycle according to said first SCR control signal and thereafteruntil said second SCR control signal is active, and to assert saidsecond background control signal during said portion of said secondhalf-cycle according to said second SCR control signal and thereafteruntil said first SCR control signal is active.
 38. The welder of claim37, wherein said flip-flop is an RS flip-flop comprising: a set inputcontrolled according to said first SCR control signal, a reset inputcontrolled according to said second SCR control signal, a first outputoperable to selectively assert said first background control signal ifsaid flip-flop is set, and a second output operable to selectivelyassert said second background control signal if said flip-flop is reset.39. A TIG welder for providing a welding current to a welding circuit,said welder comprising: a main supply comprising: a transformersecondary winding, and an SCR network including first and second SCRsconnected between said transformer secondary winding and said weldingcircuit; a control circuit connected to said first and second SCRs andoperable to provide a gating signal to one of said first and second SCRsin said SCR network to connect said transformer secondary winding tosaid welding circuit so as to provide main supply current to saidwelding circuit during a portion of a power half-cycle; and a backgroundsupply connected to said control circuit, said background supplycomprising first and second DC power supplies connected to said firstand second SCRs, respectively, wherein said background supply isoperable to provide background current to said welding circuit throughsaid one of said first and second SCRs in said SCR network during saidportion of said power half-cycle and to latch said one of said first andsecond SCRs in a conductive state thereafter so as to provide backgroundcurrent to said welding circuit after said power half-cycle until saidcontrol circuit provides a gating signal to another of said first andsecond SCRs in said SCR network.